If you're like me and you like to play in very large fields for very low stakes, then finding a good game can sometimes be a challenge. Here are a few tips from my experience :

Most sites offer multi table sit and go's. However these usually take very long to fill up on all but the largest sites.

On Full Tilt Poker you can get a 45, 90 or 180 people sng going about every hour. You need to be lucky and log on when it's half full, then it's usually 10 minutes until it's completely full and the game can begin.

On Pokerstars there are sng's in mostly 18 and 45 player formats available and these fill up extremely quickly. A 45 player tournament at Pokerstars is usually not further then 10 minutes away from you, more often you'll find yourself playing within 3 minutes. They also feature the occasional 180 player sng, and these fill up quite quickly on a good day as well.

On Party Poker, where it's definitely one of the easier sites to get a single table sng going, the multi tables seem to always get stuck under 10 players and barely ever fill up. Generally not worth my time, I only quickly look here if there is nothing else available.

Other sites where 30 or 45 player sit and gos are usually filled in around 10 minutes are VC poker, Titan Poker and CD poker (all using the iPoker network software).

So much for sng's, those fields are not large enough in my taste anyway. All poker sites also offer scheduled tournaments. This is where a tournament wolf gets his meat. These fields are so large, there will always be someone at your table willing to donk of their chips to you. In the beginning of the tourney, just play tight and try to double up early on one of the donkeys. Anyway, these tournaments are scheduled, so usually when you browse for them, they are an hour or more away. What you have to think about though, is that these tournaments can take 5 or sometimes more hours to play and even then you might not even make the final table with the big money. What I do is I play every weekend at least 1 big tournament like this. I check all the sites where I have a bankroll to see what times the big tournaments are planned for, then I plan these in iCal so I get a little warning when the registration starts. This way I usually find 2 or 3 tournaments that will run during this saturday or sunday that I'm playing.
If I have to wait an hour or maybe even half an hour, I will start a single table turbo sng to burn some time.

Sites that I can recommend for big tournaments are :
Pokerstars - definitely the biggest tournaments you'll find, with plenty of donkeys.
Full tilt poker - the tourneys here are just like the sngs, a little less visited then at pokerstars. However a few hundred players is usually the average, which is not bad in my opinion.
PartyPoker - While I usually see the largest fields here with freerolls and they don't plan many cheap large tournaments, the ones that they run are on average more then 1000 players, sometimes 2000. Definitely worth checking out if you manage to find one that fits your schedule.

So I hope this will help you decide which poker sites have the best large tournaments to play in. Enjoy and good luck at the tables !